Rolling-element bearings are used in many areas of technology to guide and support two components that rotate relative to each other. One of the components is often immobile, fixed in space, or stationary relative to a machine with which it is used or connected. The component may be, for example, the housing of the machine or another equivalent or corresponding component. In the motor vehicle field, the component may be a transmission housing, an engine block, or an axle, shaft or other wheel support. Alternately, the rolling-element bearing can be a transmission bearing (a bearing for a transmission shaft), an engine bearing (a bearing that supports a shaft or other rotating part of an engine), or a wheel bearing. But similar challenges also arise in other fields of machine, factory, and vehicle engineering, and rolling-element bearings are used in those fields as well.
In contrast to large bearings, such as are used, for example, in the wind power field, various lubricating concepts are often used in compact bearings. These bearings are often implemented as sealed bearings, bearings into which the required lubricant has been introduced during manufacturing or during assembly of the bearing. In this way lubrication can be provided for the entire service life of the bearing (for-life lubrication) or, alternatively, for a certain operating period, after which the lubricant must be changed during a servicing. Grease is often used as a lubricant in sealed bearings because grease can be more easily prevented from leaking from the rolling-element bearing than can lubricating oil.
Using grease as a lubricant is not always an optimal choice. Grease generally comprises an oil bound up or emulsified in a carrier such a soap. Under the influence of shear forces, the oil leaves the carrier and becomes available for lubrication. However, as these shear forces may only be applied to the portions of the grease near a moving surface, only a small part of the oil bound in the grease is used for actual lubrication. The remainder of the oil typically remains passive and unused in regions of the grease which are not located in the immediate vicinity of the work surfaces or contact surfaces. A not-insignificant proportion of the grease used in the rolling-element bearing thus may not contribute to lubrication, but nonetheless its presence increases the mass of a rolling-element bearing.
Liquid lubricants, on the other hand, mix and coat surfaces better than grease due to their lower viscosities, and they are used more efficiently when introduced into a rolling-element bearing. Because of their lower viscosity, however, liquid lubricants require a more expensive or complex seal to prevent them from leaking out of the rolling-element bearing.